CH 23 Digital Imaging: Exposure & Technical Factors

Technical Considerations in Digital Imaging

• Digital imaging requires changes from traditional practices, with design factors like focal spot size, SID, OID, kVp, mAs, and collimation affecting image quality
• Improving image resolution involves using smaller focal spots, shorter OIDs, longer SIDs, and shorter exposure times with higher mA

Digital Exposure Considerations

• Old exposure rules do not apply with digital detectors, requiring new considerations, such as kVp and contrast are not directly related, and mAs generally does not affect image brightness
• Total exposure incident upon the detector is what matters

Total Exposure to Detector

• Final image quality depends on total detector exposure from remnant radiation, affected by factors such as mAs, kVp, SID, OID, collimation, patient thickness, tissue composition, grids, and filters
• Higher kVp is recommended to reduce entrance skin exposure and mAs values, but mAs exposures that are too low cause quantum mottle and noise, negatively impacting image quality
• Exposure to the detector converts to an electronic data set or signal, and excessive exposure eliminates image noise but increases patient dose without visible diagnostic improvement

Exposure Indicators (EI)

• Exposure indicators monitor radiation exposure to the digital image receptor, but there is no general agreement between equipment manufacturers on calculating values
• Exposure technique systems for digital imaging should be used to ensure correct exposure factors are selected and deliver an exposure indicator within an acceptable range

Digital Exposure Technique Systems

• Technique chart systems have evolved in medical imaging's history.
• Most were developed based upon film-screen technology, where kVp and radiographic contrast as well as mAs with radiographic density had a direct relationship in technique selection
• Fixed kVp systems decrease patient dose provide more information, increase consistency, reduce x-ray tube wear, decrease time settings, and are easier to remember but produce more scatter radiation and lower contrast
• Variable kVp systems adjust 2 kVp per cm of subject thickness.
• Accurate exposure selection is essential to ALARA compliance and image quality when using CR and DR receptors, even though image processing LUTs and rescaling can compensate for exposure variations to some degree

Exposure Technique

• Establishing exposure technique systems involves using a phantom to produce test exposures and adjust either the kVp or the mAs
• The result should be reviewed with a radiologist or quality control supervisor to select the range of acceptable images so that the radiographer can choose an image from the middle

Key Steps on Establishing Exposure Technique Chart

• Phantom test exposures
• Produce range of acceptable images
• Theoretical chart extrapolation
• Clinical trial
• Clinical fine tuning
• Continuous quality assurance

Assessing Digital Exposure Technique

• Assessment of image noise is required, image contrast and brightness are determined by the exam LUTs
• The second-most common factor affecting image quality, after errors in radiographic positioning, is objectionable image noise
• Exposure indicators should be referenced to assess image noise
• Errors in technique selection have historically resulted in increased exposure technique to decrease chance of image noise and avoid the trend known as dose creep, which medical imaging experts caution is a practice that does not reflect the standards of the profession
• ASRT recommends using the highest kVp with the lowest mAs with the optimal range to provide adequate exposure to the image receptor

ALARA Compliant Imaging Departments

• Exposure technique systems use target exposure indicator values (EI) as benchmarks, and are set through discussions with the vendor, quality control experts, and department radiologists
• CR and DR work well with automatic exposure control (AEC) features
• Each exam type/projection should have an established target exposure indicator value for guidance on image quality, and a department deviation index (DI) scale established as well
• Conformance to DI values is essential to ALARA compliance

Correcting DI Numbers

• Correcting unacceptable DI numbers means that many circumstances require alterations to exposure technique calculations, like patient condition, geometric distances, x-ray tube angles, grid ratios, collimation, etc
• The display of an El value gives a reliable indication of detector exposure
• The exposure deviation index (DI) is calculated based upon target El values
• Deviation index values are included as part of the DICOM header information on images, indicating variance from established target El values

DI Values

• The American Association of Physicists in Medicine (AAPM) has outlined recommended actions related to deviation index values
• Excessive radiation exposure with a DI >+3 means repeat is required only if relevant anatomy is clipped or "burned out" and immediate management follow-up is required
• Overexposure with a DI from +1 to +3.0 means repeat is required only if relevant anatomy is clipped or burned out"
• Underexposure with a DI less than -1.0 means consult radiologist for repeat

Management Concerns

• Underexposure with negative DI values should be followed to optimize image quality and the scale of DI, and values are setup in increments of 20 percent for underexposure and 25 percent for overexposure
• Positive DI values under 3 are a cause for concern on the part of radiology managers in their efforts to optimize radiation safety

DI Values and Exposure

• DI of +2 indicates exposure to the detector was 50% above the recommended target value that would likely not jeopardize image quality, but would certainly be considered overexposure
• DI of -2 indicates underexposure to the detector that was 40% below the recommended target value, and an image that will demonstrate noise
• Exposure with DI values of +3 or higher are considered possible ALARA violations and require management follow-up on one's use of radiation

Image Quality Factors

• Image quality depends on resolution, noise, detective quantum efficiency, and artifacts
• Digital receptors do not conform to the conventional rules of radiation exposure and image quality
• Although an increase in kVp increases secondary radiation production and should reduce contrast, system software can compensate
• Decreasing the amount of scatter/secondary production, as the total amount of scatter/secondary radiation production is a function of mAs, not kVp
• Underexposure yields noisy images that may result in graininess due to photon starvation, leading to lower visual or perceived resolution and obscuring anatomical features

Setting Digital Systems

• When setting kVp levels for digital systems, when higher kVp levels are used, the width of the acquired data narrows
• Reducing patient dose by using higher kVp levels displays more anatomical data

Grid Considerations

• It's recommended that kVp levels not exceed 80 for any non-grid radiography, including the chest
• Grids become mandatory when scatter ratios approach 50%

Impact on Image Quality

• Moderate overexposure does not impact image quality and is considered professionally unacceptable and a possible violation of the ALARA principle
• Extreme overexposure can detract from optimum image quality because data elements in the detector may be overwhelmed with photon energy and become incapable of recognizing high-energy values, which is known as data drop
• Data drop can lead to detector saturation, issues for the interpreting physician, and simulates a pathological process

Image Processing

• Radiographic images are now processed by computers with software and often the final look of an image is determined by software
• Image density and contrast which were greatly affected by exposure conditions like mas and kVp with analog film-screen technology could be manipulated in order to optimize image appearance
• Image brightness, window level, and window width have been added to discussing digital image contrast, which is not greatly impact by exposure factors

Knowledge of Detail

• Digital processing includes the technologist being knowledgeable in the details of digital image acquisition, as well as post-processing parameters and procedures
• Adjusting mAs will not change the quantity of photons received by the image receptor
• Quantum mottle can not be compensated by the digital post processing so adequate mAs is required to activate digital images
• When adjusting mAs the minimum exposure index should be met to ensure that radiologists can diagnose from an image that meets the minimum data requirements of the digital system

Key Factors

• Increased latitude inherent in digital imaging provides the ability to image structures of widely different attenuation values and an increased margin for error in overexposure and underexposure to the receptor
• System should maintain a useful image brightness over a wider latitude than with film-screen
• Initial contrast that is received from the detector is low and can be displayed, demonstrating the low-contrast nature of initial information
• Soon as processing is applied, the image is displayed with the desired level with an acquired histogram and LUT

Digital Post-Processing

• A popular feature is electronic masking of a displayed image, also known as cropping or shuttering, and black overlays prevent bright extraneous display monitor light overwhelming the viewer's eyes
• Masking cannot substitute pre-exposure collimation

Imaging Annotation

• Radiographic images are produced without nomenclature regarding patient body position, laterality, respiration, mobile, etc
• Information such as body position assists the radiologist in assessing patient condition, an upright image can improve visualization of a small pneumothorax with comparison inspiration/expiration images
• An example such as patient demographics are part of the radiology information system and are electronically included as part of the patient profile, recorded on the image
• Labeling images in terms of laterality requires the use of lead, radiopaque markers, at the time of image acquisition so electronic markers are not acceptable and can be questioned legally

Electronic Marking

• With CR and DR receptors the technologist must take extra care to mark the detector correctly when lead markers are not possible, placing radiopaque markers can be a practice
• Newer flat-panel detectors now have strategically placed radiopaque identifiers as an integral detector feature, like in film-screen cassette identification windows

Image Information

• An additional consideration concerning image data is image information is added in layers according to DICOM protocol
• If the image is altered with post-processing and/or image enhancement, this new information is placed into the data set at a specific digital layer, according to software parameters
• Use markers for laterality, and be a part of the original image acquisition electronic data set
• Consistently use lead anatomic side markers on the original image when exposing

Radiographic Image Tenets

• Medical radiographs are considered a legal document
• A medical image of a patient is a pictorial record of the patient's anatomy and medical condition, as well as what they are at a single moment in the patient's medical timeline
• Radiologists assume medical images are produced in an ALARA-compliant manner

Modalities

• Medical imaging modalities are indispensable and complement each other in working up and diagnosing patient conditions
• Medical diagnostic yield (DY) is the amount of clinically significant information
• diagnostic efficacy (DE) concerns faithful representation of the patient on medical radiograph

Outside Interferences

• Hair braids or ties can simulate lung pathologies, clothing, positioning aids can show up radiographically and if not, require an explanation
• Low contrast resolution, though desirable, can present problems with image quality because clothing artifacts, tattoos, positioning aids can result in unwanted interference

Data Drop

• Extreme photon energies striking sensitive detector sensing surfaces can lead to saturation with known phenomenon

Preventing Saturation

• Preventing saturation means to understand data drop occurring from extreme photon energies requires energy control and beam collimation to reduce all energy and amount of photon energy prevents high energy

Artifacts

• Analog and digital artifacts can occur and are errors on the image
• Phantom or ghost images occur as a result of incomplete image plate erasure
• Scratches or tears are permanent and caused by damage to CR plates
• Light spots are caused by dust or foriegn material and can be avoided by recommended actions given to maintain cleanliness

Artifact Corrections

• Dropout artifacts are a result of dust that have reduced resolution and need to be cleaned according to QA maintenance
• Fogging can occur from background radiation
• Quantum (or reticulation) mottle is caused by inadequate exposure (usually insufficient mAs)

Algorithm Artifacts

• Manufacturers can restrict access or preset values in order to better control CR systems' images, in order to provide a reasonable level of consistency between images for diagnostic comparisons
• Histogram analysis error occurs from improper collimation, improper technique, beam alignment, scatter, and extreme subject density differences which can be avoided by ensuring collimatoin edges are parallel
• Electronic artifacts randomly occurs on DR images due to Bucky motor interference, x-ray tube rotor initiation, extraneous RF signals and can occur, but recognize them and have an attempt at reducing if these persist